The present invention relates generally to the generation of electrical power by a magneto-hydrodynamic (MHD) generator, and more particularly to closed-cycle MHD generators.
Power generation by means of fossil fired steam generators or boilers, which drive a steam turbine-generator, is well established. Power generation by means of magneto-hydrodynamics (MHD), or magneto-fluid-dynamics (MFD), has been used in a variety of small-scale applications.
Large-scale applications using MHD were attempted in the power generation field in the latter half of the 20th century. These applications involved trying to generate electricity using hot flue gas upstream of a fossil fired generator. The goal was to co-generate electricity directly, that is, without an intervening turbine generator, as an addition to the main quantity of electricity generated through the conventional steam turbine-generator.
As shown in FIG. 1, magneto-hydrodynamic or MHD electric power generation occurs when hot, partially ionized combustion gas (plasma) 20 is expanded through a magnetic field. Electrodes in the collection channel 10 pick up energy from the moving gas. The electrically-conducting gas 20 passing through a magnetic field produced by magnet 30 creates a voltage potential similar to moving an electrical wire through a magnetic field. This creates a direct current 40. The direct current is conditioned and inverted to alternating current 50 feeding conventional electric power distribution systems.
In traditional MHD power generation, the hot gas 20 is produced in a coal combustor at temperature approaching 5000 F. (2760 C.). Even at high gas temperatures it was generally believed to be necessary to increase the available gas ionization by seeding the gas with an easily ionized material. Potassium compounds were preferred. The spent seed compounds were treated and recycled for economic and environmental reasons.
The MHD system includes a high temperature coal combustor with seeding capability producing the high temperature plasma 20, which enters a magnetic field through a nozzle 25. The gases expand through the magnetic field and then enter a high temperature ceramic air heater 60. The high gas temperature required for the plasma 20 makes it necessary for the combustion air/oxygen to have a temperature of about 3000 F. (1649 C.). Downstream of the air heater 60, the cooled gases enter the steam bottoming portion of the plant cycle. The bottoming portion of the plant consists of a conventional steam generator capable of generating steam 100, which powers a steam turbine-generator 80.
Studies based on the cycle shown in FIG. 1 led to an advanced design 1000 MW output MHD steam plant. Advanced concepts used in this study included 3100 F. (1704 C.) direct-fired ceramic air preheaters, a high efficiency motor driven axial compressor, high pressure (1.467 kPa) combustion with low heat loss, superconducting magnets 30, low heat loss ceramic channel electrodes, high electrical stress design, moderate pressure [800 psi (1.4 Mpa)] channel cooling 10 and a high performance diffuser. The advanced concept MHD bottoming cycle resulted in a plant having a net efficiency of 60.4% on a higher heating value basis, compared to conventional cycle efficiency of around 40%. The combustion air preheat for the MHD combustor was projected to be 3100 F. (1704 C.) with a combustor pressure of 210 psi (14.5 bar). The bottoming cycle main steam throttle pressure was 5000 psi (345 bar); superheater outlet steam temperature was 1200 F. (649 C.); and reheat outlet temperatures were 1050 to 1150 F. (566 to 621 C.). The environmental impact of this cycle was projected to be significantly better in the areas of SO2, NOx, CO2, particulates, solid wastes, cooling heat rejection and total water consumption than a conventional steam plant producing the same amount of electricity.
As described above, the application of MHD for utility scale electric power generation uses MHD as a topping cycle combined with a steam bottoming cycle. This process employs a high temperature gas seeded with particles (normally potassium particles), which at high temperatures would be ionized, creating a cloud of charged particles in the hot gas. The high temperature gases were required only to keep the particles ionized and therefore electrically charged, thereby forming a plasma or cloud of charged particles. A magnetic field was to be applied across the flue carrying the ionized particles, with electric current to be taken off using contacts or terminals in contact with the gas flow normal to the direction of the magnetic field imposed. This process required operation at high gas temperatures to ensure the particles in the gases stayed ionized. In addition, due to the cost of the seed particles, highly efficient removal of the seed particles from the flue gases for reuse in the MHD process was required to make the overall process feasible economically and environmentally. This proved economically impossible to achieve to the degree necessary. The MHD topping cycle combined with a steam bottoming cycle was never commercially successful and development stopped.
More recently, others in Japan have studied development of closed-loop or closed-cycle MHD, where the gases pass through a gas turbine-generator located downstream of the MHD generator. Such a system is described in U.S. Pat. No. 5,086,234, which discloses a heat source for heating a rare gas (i.e. noble gas) and means, disposed in the heater, for adding alkali metal vapor as a seed agent to improve the conductivity of the gas. The heated rare gas and seed agent is first introduced into an MHD generator, and then discharged into a heat exchanger, where the seed agent is removed from the gas. The rare gas, with the seed agent removed, is then compressed and used to drive a gas turbine-generator unit. With such seed agent recovery, the rare gas entering the gas turbine-generator unit is substantially free from the seed agent.
U.S. Pat. No. 4,516,043 discloses an open-cycle MHD system wherein carbon particles serve as charge carriers, and which are transported by combustion gases in the MHD process. A main combustor produces a flow of hot gases containing microscopic carbon particles, the particles preferably having a mean diameter between about 0.02×10−6 m to 0.04×10−6 m (20-40 nanometers). The flow of hot gas, together with the entrained carbon particles, is directed into an electrostatic charging device that positively charges the carbon particles. The charging device may be essentially similar to the electrostatic precipitators used to remove fly ash and other particulates from stack gases. The combustion gases and charged charge carriers are directed into an MHD generator, and are then discharged to an afterburner where the carbon particles are removed from the flow by combustion. The MHD generator is operated at temperatures between about 1500 degrees C. and 2500 degrees C. The heat in the combustion gases, including heat produced from combustion of the carbon particles, is used to generate steam to run a steam turbine-generator in a steam bottoming cycle. The goal of the above arrangement is to provide a less costly MHD topping cycle that does not require seeding with alkali metals.
The MHD systems described above use high temperature combustion gas or noble gases as the working fluid to transport charge carriers through the MHD generator. The MHD generator, when combined with a steam or gas turbine to improve cycle efficiency, is located upstream of the turbine-generator as part of an MHD topping cycle. The seed material is either removed from the working fluid and then returned to the working fluid in closed-cycle systems, or is discharged from the system in open-cycle systems.